The acetoacetate moiety has been used in the coatings industry to impart functionality capable of undergoing a variety of cross-linking reaction while simultaneously lowering the viscosity of the resulting formulation. Among the reactions which can be used to promote cross-linking of acetoacetylated polymeric materials are reactions with activated olefins (commonly referred to as the Michael reaction), diamines, melamine, isocyanates, and the like. Coatings prepared from acetoacetylated polymers using such cross-linking strategies often exhibit improved stain-resistance, salt-spray resistance and better adhesion to the metal surface when compared to coatings prepared from nonacetoacetylated polymers.
Interest in the use of acetoacetylated materials in coatings has led to the need for general synthetic procedures for the preparation of acetoacetylated compounds which can be readily practiced on industrial scale. It is known that acetoacetylated acrylic resins can be prepared by the copolymerization of acetoacetoxyethyl methacrylate with acrylic or methacrylic monomers. Alternatively, acetoacetylated polymers or resins can be prepared by the acetoacetylation of the polymeric substrate, rather than by polymerization of acetoacetylated monomers. One substrate for which this method of synthesis is generally required is in the preparation of acetoacetylated polyester resins.
Simple acetoacetylated materials can be prepared in a variety of ways. For example, an appropriate nucleophile can be treated with diketene. Alternatively, such nucleophile can be subjected to a thermal reaction with 2,2,6-trimethyl-4H-1,3-dioxin-4.one (TKD, the diketene-acetone adduct). As yet another alternative, such nucleophile can be subjected to transesterification with another acetoacetate moiety (referred to hereinafter as "transacetoacetylation").
The industrial-scale use of diketene for such applications is impractical due to strict governmental regulations regarding the shipping of this material. In addition, the classification of diketene as a lachrymator make the large scale use of this material undesirable. The dioxinone, TKD, while effective for acetoacetylation, is currently too costly a raw material to be employed for large scale industrial applications.
While transesterification reactions are well known in the preparation of polyester coating resins, transesterification of acetoacetates (i.e., transacetoacetylation) has not found wide spread application. One published procedure for the preparation of acetoacetic acid derivatives involves heating solutions of a higher boiling alcohol with an excess of methyl or ethyl acetoacetate while the volatile methyl or ethyl alcohol co-product is removed by distillation. Reaction times for such procedure are on the order of many hours when carried out at elevated temperatures (about 100.degree. C). Another method for transacetoacetylation which has been suggested in the art involves contacting the alcohol of interest with a large excess of methylacetoacetate and a 4-dimethylaminopyridine catalyst in a high boiling hydrocarbon solvent such as toluene for an extended period of time. An alternate method for transacetoacetylation disclosed in the art is the use of titanium catalysts.
Seebach et al, Synthesis, 1982, pages 138-141, disclose the use of substantial amounts of titanium (IV) alkoxides (tetraalkyl titanates) as catalysts in the transacetacetylation of alcohols. More specifically, this reference discloses the reaction of tertiary (t) butyl acetoacetate with n-butanol and benzyl alcohol in the presence of tetraethyl titanate wherein the molar ratio of the titanate used to t-butyl acetoacetate was 0.21 or 0.35.
Yet another prior art disclosure of transesterification reactions employing acetoacetic moieties is found in European Patent Application 227,454, assigned to Cook Paint and Varnish Inc. The reaction between a polyhydroxy functional monomer or polymer and an alkyl monofunctional acetoacetate is disclosed. Suitable acetoacetate esters are disclosed to be methyl acetoacetate, ethyl acetoacetate, isopropyl acetoacetate, butyl acetoacetate, t-butyl acetoacetate, methyl benzyl acetoacetate and dodecyl acetoacetate. The examples in this disclosure demonstrate only the use of ethyl acetoacetate. There is no suggestion in the reference of any benefit from using one acetoacetate moiety rather than another taken from the above list of "suitable" compounds.
The high dilution, large amounts of catalyst used and long reaction times involved make each of the prior procedures for transacetoacetylation impractical, especially when application on a commercial scale is contemplated. Prior art procedures are particularly ill-suited for the acetoacetylation of higher molecular weight (including polymeric) nucleophiles. There is, therefore, a need in the art for simplified procedure for transacetoacetylation, which procedure does not require extreme reaction conditions or large quantities of unreactive materials.